Carbon and graphite fibers and composites made therefrom are increasingly used in the manufacture of components for lightweight aircraft, aerospace structures, automobile parts, and sporting equipment. Carbonaceous material is melted, spun into thread or filament form, and converted to a carbon or graphite fiber. The spun filament or filaments are stabilized, i.e. rendered infusible through a heat treatment in an oxidizing atmosphere, and thereafter heated to a higher temperature in an inert atmosphere to convert the filament into a carbon or graphite fiber.
A significantly large percentage of commercial carbon fiber processes employ mesophase pitch as the source of the carbon or graphite fiber. The high cost of the graphite fibers so produced is due primarily to the cost of producing the mesophase pitch as the base of such fiber manufacture. Further, most of the commercial fibers produced from mesophase pitch has been fibers which have been subsequently converted to graphite fibers. Because the temperature of graphitization is higher than the temperature required to produce a carbon fiber, graphite fibers are much more costly to produce than carbon fibers. Attempts have been made to manufacture carbon fibers from pitch materials without converting the pitch into the mesophase state.
Canadian Pat. No. 1,177,605 issued Nov. 12, 1984, to the common corporate assignee, is directed to the production of non-mesophasic aromatic enriched pitches which can be quickly processed into carbon fibers at a much lower cost having excellent intermediate properties, permitting them to be used in many applications where asbestos is being currently used. As a result thereof, such carbon fibers can be employed in the manufacture of brake drums and discs, particularly useful in the automotive field. Such fibers also constitute an excellent replacement for asbestos fiber.
Referring to FIG. 1, there is shown a flow diagram covering the process for the manufacture of carbon fibers from non-mesophase pitch as exemplified by Canadian Pat. No. 1,177,605 and to which process the present invention has application. In order to appreciate the content of the present invention as directed to improvements in the formation of the carbon fibers and their subsequent stabilization and carbonization and for facilitating the stabilizing and carbonizing process steps, a brief review of the nature of manufacture of pitch carbon fibers from FIG. 1 is necessary. The content of Canadian Pat. No. 1,177,605 is incorporated by specific reference herein. The starting petroleum pitch utilized in the process of this invention, as in the case of Canadian Pat. No. 1,177,605, may be an aromatic base, unoxidized carbonaceous pitch produced from heavy slurry oil produced in the catalytic cracking of petroleum distillates. As such, in FIG. 1, block 10 represents the fluid catalytic cracker, and the slurry oil produced in the catalytic cracking of petroleum distillates is fed through closed pipeline 12 to a pitch unit 14 for further cracking and processing producing a type of pitch under the Ashland Oil, Inc., designation A-240, which is supplied to the wiped-film evaporator 18 via closed pipeline 16. Such pitch is a commercially available unoxidized petroleum pitch meeting the requirements of the low cost production of non-mesophase pitch carbon fiber. The term "non-mesophase" is meant to means less than 5% by weight of mesophase pitch. Such a pitch is generally referred to in the art as an isotropic pitch, e.g. a pitch exhibiting physical properties such as light transmission with the same values when measured along axes in all directions. In a process of the referred to Canadian patent, the wiped film evaporator is used to reduce the time of thermal exposure of the product, thus producing a better fiber precursor. The evaporator 18 may be of the type manufactured by Artisan Industries, Inc. of Waltham, Massachusetts and sold under the trademark Rothotherm. In such a wiped-film evaporator, the feed, i.e. the A-240 pitch material, enters the unit and is thrown by centrifugal force against the heated evaporator walls to form the turbulent film between the wall and rotor blade tips. The turbulent flowing film covers the entire wall, regardless of the evaporation rate. The material is exposed to high temperature for only a few seconds. Briefly, as described in the Canadian patent, A-240 pitch material is melted in a melt tank after being filtered to remove contaminants including catalyst fines. It is pumped through a back pressure valve into the wiped-film evaporator 18. In turn, the wiped-film evaporator is heated by hot oil contained in a reservoir which is pumped into the thin film evaporator through a supply line. As the A-240 pitch material is treated in the thin film evaporator, vapors escape the evaporator and are condensed in first and second condensers. The vapors then pass through a conduit into a cold trap and out through a line with vacuum being applied to the system via a vacuum pump. Under these conditions, feed rates of between 15 to 20 pounds of A-240 pitch per hour are utilized which produce about 10 pounds per hour of a higher softening point pitch, in turn, supplied by the wiped-film evaporator 18 to the fiber forming apparatus indicated at 20 via a further closed pipeline 22. Preferably, the fiber forming apparatus 20 is a melt blowing extruder of the type disclosed in U.S. Pat. Nos. 3,615,995 and 3,684,415 to Buntin. The melt blowing extruder operates such that the high softening point pitch fed thereto is extruded through a large number of orifices of suitable diameter into a moving stream of hot inert gas which issues from outlets surrounding or adjacent to the orifices so as to attenuate the molten material into fibers which form a fiber stream. The hot inert gas stream flows at a linear velocity parallel to and higher than the filaments issuing from the orifices, so that the filaments are drawn by the gas stream, entrained therein and moved therewith. The fibers in the process of Canadian Pat. No. 1,177,605 are collected on a receiver in the path of the fiber stream to form a non-woven mat. Arrow 22 from the fiber forming block or apparatus 20 of the flow diagram represents the receiver. The fibers borne by the receiver (or the fibers after removal from the receiver), are then subjected to stabilization within an enclosed stabilizer 24.
The fibers made from the pitch are successfully stabilized in air by subjecting the fibers to a special heat cycle. As set forth within Canadian Pat. No. 1,177,605, the stabilization process is effected in less than 100 minutes, with the 100 minute cycle consisting of holding the pitch fibers at approximately 11.degree. C. (20.degree. F.) below the glass transition temperature (Tg) of the precursor pitch i.e. about 180.degree. C. (356.degree. F.) for about 50 minutes. This is followerd by an increase to about 200.degree. C. (392.degree. F.) and holding 30 minutes at that temperature. The temperature is then increased to about 265.degree. C. (509.degree. F.) and the fibers held 10 minutes. Finally, the fibers are heated to about 305.degree. C. (581.degree. F.) and held 10 minutes at this temperature.
Thereafter, the fibers in non-woven mat form, either on the receiver or removed from the receiver, are then subjected to a carbonizing process in a carbonizer, as evidenced in block form at 28 in the flow diagram of FIG. 1. This may be a separate enclosure 28, as in FIG. 1, or the stabilizer enclosure 24, subjected to different operating parameters. The transfer is schematically shown by arrow 26 in FIG. 1. The carbonization step involves a modification of the physical properties of the fibers after further heating to about 1100.degree. C. (2000.degree. F.) in an inert atmosphere such as nitrogen for several hours (two hours) in order to convert them to carbon fibers. Subsequently, they are removed and given final product packaging as exemplified by block 32 with the removal step being shown by arrow 30.
It should be kept in mind that the term "oxidizing" environment means either subjecting the fibers to an oxidizing atmosphere or impregnating an oxidizing material within or on the surface of the individual fibers. An oxidizing atmosphere may consist of a gas such as air, enriched air, oxygen, ozone, nitrogen oxide, sulfur oxide, etc. The impregnated oxidizing material can be one of any of a number of oxidizing agents such as sulfur, nitrogen oxides, sulfur oxides, peroxides, persulfates, etc. Stabilization of fibers made from other high softening point pitches, such as an A-410-VR pitch, involves similar heating cycles for an extended period of time, i.e. 36 hours, with similar step increases in temperature. It should be kept in mind that if either temperature is exceeded or time shortened, the fibers begin to melt and fuse during subsequent processing.
In the process of Canadian Pat. No. 1,177,605, it was found that air stabilization is much more effective where the fibers are first heated to a temperature of about 6.degree. to 11.degree. C. (10.degree. to 20.degree. F.) below the glass transition temperature of the pitch precursor and thereafter, after a period of time of approximately 50 minutes, heated to a temperature within the range of 299.degree.-316.degree. C. (570.degree. to 600.degree. F.) until stabilization is reached. The glass transition point represents the temperature of Young's modulus change and is also the temperature at which the glassy material undergoes a change in coefficient of expansion which is often associated with a stress release. At 6.degree. to 11.degree. C. below the glass transition temperature, the fibers maintain their stiffness while at the same time the temperature represents the highest temperature allowable for satisfactory stabilization to occur. This temperature is below the point at which fiber-fiber fusion can occur. After the fiber has been heated at this temperature for a sufficient time to form a skin, the temperature can be raised at a rate such that the increased temperature is below the glass transition temperature of the oxidized fibers, protected against fusion by the skin. It was further discovered that during the oxidation of the carbon fibers, a glass transition temperature increases and by maintaining the temperature during heat up at a point 6.degree. C. below the glass transition temperature, undesired slumping of the fibers does not occur. As the temperature is increased, the oxidation rate increases and conversely the stabilization time decreases.
While the process as set forth in Canadian Pat. No. 1,177,605 produces petroleum pitch based carbon fibers in which the fibers are prepared by melt blowing, and wherein the fibers are collected on a receiver positioned in the path of the air blow fibers to create a non-woven mat, in the manner of the U.S. Pat. Nos. 3,615,995 and 3,684,415 to Buntin, the resultant non-woven mat creates an end product which is undesirable and has limited utility.
By melt blowing, the individual fibers are not only reduced in diameter, but accelerated to velocities in terms of hundreds of miles per hour. Buntin collects the fibers in an effort to form a relatively thick mat. As such, the fibers are non-aligned, and the mat is achieved by slowly rotating a small diameter cylinder or wheel over a limited arc. The result of this is that the fibers impinge the periphery of the cylinder at 100 miles per hour or so, while the wheel is turning such that its periphery moves approximately one foot per minute, i.e. 60 feet per hour. As a result, there are hundreds of miles of multiple fibers piled up in 60 feet. As a result, Buntin creates a mat of non-aligned thermoplastic polymer fibers, as the wheel continues to turn, with the piled fiber mat being pulled off after the fiber mat moves approximately 1/4 of a rotation in contact with the screen drum upon which the fibers impinge. As may be appreciated, this has the net result of frustrating both the stabilization and carbonization processes required of the carbon fibers and the creation of a non-woven fiber mat comprised of aligned fibers.